Engine out cold-start emissions generated before light-off of an exhaust system catalytic converter may contribute a large percentage of the total exhaust emissions. Various approaches may be used by engine control systems to expedite the attainment of the catalyst light-off temperature. For example, various combinations of valve overlap and increased fuel injection may be used to expedite catalyst warming. In addition, at low engine speeds following the cold-start, boost performance may be degraded due to turbo lag. In boosted engine systems including a twin-scroll turbocharger, such as the system of Styles et al. in US 2011/0302917, flow through the different scrolls of the turbocharger is separated to reduce turbo lag at low engine speeds.
However, the inventors herein have recognized potential issues with such an approach. Specifically, there may be selected engine operating conditions where separating flow through the scrolls of a twin-scroll turbocharger can increase turbine speed to a range that excites audible resonance at the turbocharger. This results in a condition known as turbo whine. The audible resonance may be objectionable to the vehicle driver and/or passengers. The turbine speed at which the resonance occurs may vary based on operating conditions such as engine speed, temperature, etc., the turbo whine may be exacerbated during low engine speed conditions, for example during engine start. This is due to the overall engine system noise not being sufficient to mask the audible resonance.
In one example, turbo whine may be at least partly addressed by a boosted engine system that uses a binary flow turbine. For example, turbo whine may be addressed by a method comprising: during selected engine start (e.g., engine start speed conditions) conditions, operating the engine with a scroll valve coupled to an outer scroll of a multi-scroll exhaust turbine open. Specifically for a start event, by opening the scroll valve during at least an early part of selected engine starts, turbine speed can be reduced, moving the turbocharger out of the resonance condition. During other starts, where no whine occurs, the engine may be started with the scroll valve closed for a number of combustion events.
In one example, during selected engine starts, such as where the turbine speed is within a threshold range, an engine may be started with a scroll valve coupled to an outer scroll of a multi-scroll exhaust turbine at a more open position, such as at a fully open position. The scroll valve may be maintained open for a number of combustion events during the engine start. By initially opening the scroll valve, exhaust manifold pressure upstream of the turbine is reduced, reducing turbine speed. This may move the turbine out of a speed range where resonance occurs, thereby reducing the objectionable noise. Then, after the number of combustion events, the scroll valve may be moved to a closed position to expedite turbine spool-up and catalyst warming. By subsequently closing the scroll valve, exhaust flowing to the turbine inlet is flowed at a higher pressure, expediting turbine spin-up and resulting turbine power. In addition, turbine surface area is reduced, increasing heat flux to the exhaust catalyst, and expediting catalyst light-off during the cold-start. It will be appreciated that wastegate and EGR valve adjustments may be coordinated with the scroll valve adjustment to further improve boost control during the engine start.
In this way, by opening the scroll valve during an early part of an engine start, turbine speed can be kept out of a range where turbo whine is likely. Then, by closing the scroll valve during a later part of the engine start, turbine spin-up and catalyst light-off is expedited. By reducing turbo whine, the vehicle operator's drive experience is improved. By expediting catalyst light-off, cold-start emissions are reduced. In addition, by expediting turbine spin-up, turbo lag at low engine speeds is reduced, and boosted engine performance is improved. Overall, engine performance and exhaust emissions are improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.